Reuse base class code

[Pavel Ivanov]

The abstract class Message, among others, defines two protected properties - $headers & $body. Any derived class will be inherit them. In paticular, two classes Request & Response. But classes define constructor parameters differently.

• Request::__construct(..., HeadersInterface $headers, StreamInterface $body)
• Response::__construct(..., ?HeadersInterface $headers, ?StreamInterface $body)
  If the base class (Message) defines the constructor, what would be the default behavior ?
  ?HeadersInterface or HeadersInterface & ?StreamInterface or StreamInterface.

[Pierre B.]

I think we should make it optional for both ::__construct(..., ?HeadersInterface $headers, ?StreamInterface $body)